sig4

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  Row and column counts
  
  Count the number of nonzero cells 
    in each column of A and
    in each row    of A.
    
  Arrange the two sets of 9 digits each
    in nondescending order.
    
  If A is not rotated, the row count and the
    column count are the same before and after
    a transformation .
    
  If A is rotated 90 degrees, then the
    row and column counts reverse.
    
  Adopt the convention that the lower of the
    two numbers is always displayed first.
    
  Then the row-column pair is a signature.